Method for fabricating transfer printing substrate using concave-convex structure, transfer printing substrate fabricated thereby and application thereof

ABSTRACT

A method for fabricating a substrate for transfer printing using a concave-convex structure and a substrate for transfer printing fabricated thereby. The method includes preparing a handling substrate having a concave-convex structure formed thereon; forming a sacrificial layer along the concave-convex structure on the handling substrate; coating a polymer on the handling substrate having the sacrificial layer formed thereon to form a polymer substrate having bumps filling a concave portion of the concave-convex structure; and removing the sacrificial layer from the handling substrate. The substrate includes a handling substrate having a concave-convex structure formed thereon; and a polymer substrate placed on the concave-convex structure and having bumps filling a concave portion of the concave-convex structure of the handling substrate. This process of manufacturing provides a device to be stably performed on an ultrathin substrate and may secure high degree of alignment and high transfer yield in a transfer printing process.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application Nos.10-2011-0137726 and 10-2012-0097372 filed on Dec. 19, 2011 and Sep. 3,2012, and all the benefits accruing therefrom under 35 U.S.C. §119, thecontents of which are incorporated by reference in their entirety.

BACKGROUND

1. Technical Field

The present invention relates to a method for fabricating a substratefor transfer printing and a substrate for transfer printing fabricatedthereby. More particularly, the present invention relates to a methodfor fabricating a substrate for transfer printing using a handlingsubstrate having a concave-convex structure, a substrate for transferprinting fabricated thereby, and application thereof

2. Description of the Related Art

As future electric/electronic devices are required to be mounted notonly on simple planar surfaces, but also on various surfaces, such asskin surfaces, clothes surfaces, interior or exterior surfaces ofbuildings, body surfaces of persons, and the like, the devices need tohave highly mechanical flexibility. Since flexibility of the devices canbe easily obtained through a smaller thickness thereof, substrates usedfor the devices are required to have as small a thickness as possible.However, when the thickness of the substrate decreases to severalmicrometers or less, the substrate inevitably undergoes twisting orother deformation due to mechanical flexibility of the substrate in thecourse of processing. Thus, a handling substrate is used to support anultrathin substrate in order to secure process stability. In this case,however, a process of transferring a thin film type device formed on theultrathin substrate through separation from the handling substrate totransfer the device to another surface is required. At this time,alignment and transfer yield are very important. In this regard, it isimportant to take three factors into account: first, the ultrathinsubstrate must retain mechanical stability in the manufacturing process;secondly, an initial arrangement pattern of the ultrathin substrate mustbe maintained in the manufacture and transfer printing processes of thedevice; and thirdly, transfer printing must be performed at high yield.

In one example of transfer printing, an ultrathin substrate is directlycoated on the handling substrate. Although this method can provideprocess stability, there can be difficulty in transfer of the ultrathinsubstrate to another substrate due to high adhesion force between thehandling substrate and the ultrathin substrate. Accordingly, it can becontemplated that a device is fabricated on the ultrathin substrate,with a sacrificial layer interposed between the handling substrate andthe ultrathin substrate, and is then transferred after removing thesacrificial layer from the handling substrate. However, when removingthe sacrificial layer using an etching solution, the pattern of theultrathin substrate placed on the handling substrate floats causing lossof initial arrangement, and sinks into the handling substrate causingdeterioration of transfer yield. Meanwhile, in another method, aninsulation layer can be interposed between the handling substrate andthe ultrathin substrate to allow the device to be separated from thehandling substrate upon laser irradiation, causing increase inmanufacturing cost and a possibility of damaging the device by a laserbeam. Moreover, this method suffers from performance deterioration ofthe device due to formation of an uneven surface upon detachment fromthe handling substrate.

Therefore, there is a need for a method of manufacturing a device on anultrathin substrate, which can maximize the degree of alignment andtransfer yield of the device in the course of transfer printing, whilesecuring process stability.

BRIEF SUMMARY

Therefore, the present invention is aimed at providing a substrate fortransfer printing and a method of manufacturing the same, which areuseful in realization of high degree of alignment and transfer yieldwhile minimizing damage of devices.

One aspect of the present invention provides a method for fabricating asubstrate for transfer printing. The method for fabricating a substratefor transfer printing includes: preparing a handling substrate having aconcave-convex structure formed thereon; forming a sacrificial layeralong the concave-convex structure on the handling substrate; coating apolymer on the handling substrate having the sacrificial layer formedthereon to form a polymer substrate having bumps filling a concaveportion of the concave-convex structure; and removing the sacrificiallayer from the handling substrate.

Another aspect of the present invention provides a substrate fortransfer printing. The substrate for transfer printing includes ahandling substrate having a concave-convex structure formed thereon; anda polymer substrate placed on the concave-convex structure and havingbumps filling a concave portion of the concave-convex structure of thehandling substrate.

A further aspect of the present invention provides a method forfabricating an electronic device for transfer printing. The method forfabricating an electronic device for transfer printing includes:preparing a handling substrate having a concave-convex structure formedthereon; forming a sacrificial layer along the concave-convex structureon the handling substrate; coating a polymer on the handling substratehaving the sacrificial layer formed thereon to form a polymer substratehaving bumps filling a concave portion of the concave-convex structure;and removing the sacrificial layer from the handling substrate, whereinan electronic device is formed on the polymer substrate before or afterremoving the sacrificial layer.

According to the present invention, since the polymer substrate isstructurally bound to the handling substrate having a concave-convexstructure formed thereon, arrangement of the polymer substrate can befirmly maintained even in the case of removing the sacrificial layerfrom between the polymer substrate and the handling substrate.

In addition, even in the case where the sacrificial layer is removed,the handling substrate is not in complete contact with the polymersubstrate to reduce adhesion force therebetween, thereby allowing easytransfer to a transfer medium.

Further, the process of manufacturing a device can be stably performedon the polymer substrate, and, when components of the device isvulnerable to an etching solution used in removal of the sacrificiallayer, the process of removing the sacrificial layer may be performedbefore manufacturing the device to prevent damage of the device.

Meanwhile, a water soluble polymer may be employed as a material for thesacrificial layer and a non-toxic material such as water may be employedas an etching solution. In this case, there will be no problem of damageof the polymer substrate and the device due to the etching solution. Asa result, the process of removing the sacrificial layer may be performedafter manufacturing the device on the polymer substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will become apparent from the detailed description of thefollowing embodiments in conjunction with the accompanying drawings, inwhich:

FIGS. 1 to 5 are perspective views and sectional views illustratingsequences of a method for fabricating a substrate for transfer printingaccording to one embodiment of the present invention;

FIG. 6 is a perspective view of a process of transferring a polymersubstrate from a handling substrate to a transfer medium;

FIG. 7 is a perspective view of a process of printing the polymersubstrate from the transfer medium to a target substrate;

FIG. 8 is a schematic diagram of a process of manufacturing a substratefor transfer printing according to Example 1 and SEM images of resultingproducts;

FIGS. 9 and 10 are graphs depicting a degree of alignment and transferyield of a polymer substrate depending on the shape, depth and pitch ofa concave portion of a concave-convex structure;

FIG. 11 shows optical micrographs of polymer substrates transferred to atransfer medium and polymer substrates printed on target substrates;

FIG. 12 is a picture of organic light emitting devices prepared inExample 4;

FIG. 13 is a schematic flow diagram of a process of manufacturing andtransferring electronic devices for transfer printing in Example 5;

FIG. 14 shows SEM images of resulting products in each step of theprocess in Example 5;

FIGS. 15 a and 15 b are graphs depicting degree of alignment (AD) andtransfer yield (TY) measured in Example 6;

FIG. 16 is a graph depicting degree of alignment (AD) and transfer yield(TY) measured in Example 7 and Comparative Example 1;

FIG. 17 shows optical micrographs of polymer substrates transferred totransfer media in Example 8;

FIG. 18 shows pictures and graphs depicting characteristics of devicesfor transfer printing, which includes a ZnO-TFT array according toExample 9; and

FIG. 19 shows a picture of a ZnO-TFT array transfer-printed on a PETfilm and graphs depicting variation of characteristics of the ZnO-TFTarray depending on the degree of bending.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will now be described indetail with reference to the accompanying drawings. However, it shouldbe noted that the present invention may be embodied in various differentforms without being limited to the illustrated embodiments and isintended to embrace all equivalents and substitutions that fall withinthe spirit and scope of the appended claims. In the drawings, thethicknesses of layers and regions can be exaggerated or omitted forclarity. The same components will be denoted by the same referencenumerals throughout the specification. Furthermore, descriptions ofdetails apparent to those skilled in the art will be omitted forclarity.

FIGS. 1 to 5 are perspective views and sectional views illustratingsequences of a method for fabricating a substrate for transfer printingaccording to one embodiment of the present invention.

Referring to FIGS. 1 and 2, a photoresist layer 102 having a pattern ofplural holes (h) is formed on a handling substrate 100 (FIG. 1). Thehandling substrate 100 is etched along the holes (h) on the photoresistlayer 102 and the remaining photoresist layer is removed to form aconcave-convex structure 108 on the handling substrate 100 (FIG. 2).

The handling substrate 100 may be a glass or silicon substrate, withoutbeing limited thereto. The photoresist layer 102 may be formed by anytypical photolithography, and the handling substrate 100 may be etchedby various etching methods according to the kind of substrate. Forexample, when the handling substrate 100 is a glass substrate, anetching solution such as BOE (Buffered Oxide Etchant) may be used. Whenthe handling substrate 100 is a silicon substrate, dry etching, forexample, reactive ion etching (RIE), may be used. In particular, whenthe handling substrate 100 is etched by reactive ion etching, there isan advantage in that the concave-convex structure 108 can be rapidlyformed to a desired shape and size, since physical etching and chemicaletching are performed at the same time.

In the concave-convex structure 108, a concave portion 104 may be formedin various shapes. For example, the concave portion 104 may havecircular, elliptical, polygonal and other geometrical cross-sections(that is, a cross-section of the handling substrate 100 in a horizontaldirection). In addition, the concave portion 104 may have a pot-shapedlongitudinal cross-section (that is, a cross-section of the handlingsubstrate 100 in a vertical direction), an inlet of which is smallerthan a belly thereof.

In operation of preparing the handling substrate 100 having aconcave-convex structure 108 thereon, the concave-convex structure 108may be formed on the surface of the handling substrate 100 using a maskfor etching together with the photoresist layer 102 described above.Alternatively, a handling substrate 100 having the concave-convexstructure 108 previously formed thereon may be used.

Referring to FIG. 3, a sacrificial layer 110 is formed on the handlingsubstrate 100 along the concave-convex structure 108. Specifically, thesacrificial layer 110 is formed along morphology of the concave-convexstructure 108 and does not substantially change the shape of the concaveportion 104.

The sacrificial layer 110 may be an inorganic or organic thin film. Forexample, the sacrificial layer 110 may be a thin film comprised of amaterial selected from inorganic oxides such as SiO₂ and phosphorsilicate; polymers such as polyacrylic acid (PAA), polymethylmethacrylate (PMMA), polystyrene (PS) and dextran; and metals such asnickel, copper, chromium, titanium, silver, and aluminum. Thesacrificial layer 110 may be formed by a deposition process such assputtering, PECVD, electronic-beam deposition, and thermal deposition,or a solution process such as spin coating or spray coating, accordingto the material used for the sacrificial layer.

The sacrificial layer 110 formed on an upper surface of the convexportion 106 of the concave-convex structure may have a higher thicknessthan that of the sacrificial layer 110 formed on a lower surface of theconcave portion 104 of the concave-convex structure. Such a thicknessdifference occurs since it is more spatially disadvantageous for a rawmaterial for forming the sacrificial layer 110 to approach the lowersurface of the concave portion 104 of the concave-convex structure thanthe upper surface of the convex portion 106 of the concave-convexstructure.

Referring to FIG. 4, a polymer is coated on the handling substrate 100,which has the sacrificial layer 110 formed thereon, to form a polymersubstrate which has bumps 122 filling the concave portion 104 of theconcave-convex structure while covering the handling substrate 100. Aprocess of coating the polymer may be performed by wet coating such asspin coating using a solution containing the polymer. In the course ofcoating the polymer, the concave portion 104 of the concave-convexstructure is filled with the polymer, which in turn forms the bumps 122having a shape corresponding to the shape of the concave portion 104.

The polymer may be selected from polyethylene terephthalate (PET),polyethersulfone (PES), polystyrene (PS), polycarbonate (PC), polyimide(PI), polyethylene naphthalate (PEN), polyarylates (PAR), SU-8, and thelike, without being limited thereto.

The polymer substrate may be subjected to patterning to have a desiredshape and arrangement. In one embodiment, the polymer substrate may beused as a parent substrate, which will be divided into a plurality ofpolymer substrates 120 having a smaller size than the parent substratethrough patterning. In this case, each of the polymer substrates 120includes at least one bump 122 inserted into the concave portion 104 ofthe concave-convex structure and may be maintained at an original placein a subsequent process for removing the sacrificial layer 122 ormanufacturing a device on the polymer substrate 120, as described below.The patterning process may be suitably carried out by any typical methodknown in the art according to the characteristics of the material forthe polymer substrate 120. For example, when a linear polymer such as PIis used for the polymer substrate, reactive ion etching may be employed,and when an optical cross-linkable polymer such as SU-8 is used for thepolymer substrate, photolithography may be employed.

Referring to FIG. 5, the sacrificial layer interposed between thehandling substrate 100 and the polymer substrate 120 is removed. Removalof the sacrificial layer 110 may be achieved by wet etching. The wetetching may be performed using any etching solution capable ofselectively removing the sacrificial layer without damaging the polymersubstrate 120. For example, the etching solution may be suitablyselected from water, HF solution, acetone, FeCl₃ solution, sodiumpersulfate solution, toluene, and a mixture of phosphoric acid/nitricacid/acetic acid/water.

In this case, since the bumps 122 of the polymer substrate 120 areinserted into the concave portion 104 of the handling substrate 100 evenafter removal of the sacrificial layer 110, the polymer substrate 120 isnot deviated from the original place thereof. Thus, the arrangementpattern of the polymer substrates 120 can be maintained.

In particular, when the concave portion 104 of the concave-convexstructure has a pot-shaped longitudinal cross-section, the inlet ofwhich is smaller than the belly thereof, the location of the polymersubstrate 120 can be more firmly maintained. That is, a neck of theconcave portion 104 of the concave-convex structure corresponding to aportion between the belly and the inlet of the concave portion can holdthe belly of the convex portion 122 of the polymer substrate 120 (thatis, a portion having the largest circumference of the convex portion)corresponding to the shape of the concave portion 104.

Meanwhile, irrespective of the longitudinal cross-sectional shape of theconcave portion 104, when a polymer such as polyacrylic acid (PAA) anddextran is used as a material for the sacrificial layer 110, the polymersubstrate 120 may be held by adhesion of the polymer constituting thesacrificial layer 110. In the process of etching the sacrificial layer110, the polymer of the sacrificial layer 110 placed within the concaveportion 104 of the convex-concave structure is etched at a slower ratethan the polymer of the sacrificial layer 110 placed in other regionsthereof, whereby the polymer can be concentrated on and remain in theconcave portion 104 of the concave-convex structure upon drying. Such aphenomenon may occur due to topological confinement of the remainingsacrificial layer polymer within and around the concave portion 104 ofthe concave-convex structure. The remaining polymer of the sacrificiallayer 110 may hold the polymer substrate 120 with suitable adhesion.Thus, since the remaining amount of the polymer of the sacrificial layer110 may be adjusted by regulating the degree of etching of thesacrificial layer 110, it is possible to easily adjust adhesion betweenthe handling substrate 100 and the polymer substrate 120.

In addition, since the polymer substrate 120 is prepared by the processof forming the polymer substrate on the sacrificial layer 110 andremoving the sacrificial layer 110, instead of the process of directlyforming the polymer substrate on the handling substrate 100, adhesionbetween both substrates 110, 120 can be lower than the adhesiontherebetween in the case of directly forming the polymer substrate 120on the handling substrate 110.

Further, even in the case where the sacrificial layer 110 is removed,the polymer substrate 120 does not sink into the handling substrate 100to be brought into complete contact therewith, and at least part of thepolymer substrate 120 having contacted the sacrificial layer 110 may bepresent in a state of being separated from the handling substrate 100.This is because the polymer substrate 120 can be floated by the etchingsolution present in a space from which the sacrificial layer 110 isremoved. However, even in the case where the etching solution is removedby drying, the polymer substrate 120 is supported by the bumps 122, forexample, when the bumps 122 of the polymer substrate 120 have a greaterheight than the depth of the concave portion 104 of the concave-convexstructure (this structure can appear when the thickness of thesacrificial layer 110 formed on the upper surface of the convex portion106 of the concave-convex structure is higher than that of thesacrificial layer 110 formed on the lower surface of the concave portion104 of the concave-convex structure, as described with reference to FIG.3), whereby a separation space can be defined in at least some regionbetween the convex portion 106 of the concave-convex structure and thepolymer substrate 120. In another example, the belly of the bump 122 ofthe polymer substrate 120 is caught by a certain portion of the concaveportion 104 of the concave-convex structure having a smaller size thanthe belly of the bump 122, thereby preventing the polymer substrate 120from completely sinking into the handling substrate 100. In a furtherexample, when a polymer is used as the material for the sacrificiallayer 120, the sacrificial layer 120 is not completely removed byetching and remains within or around the concave portion 104 of theconcave-convex structure, thereby preventing the polymer substrate 120from sinking into the handling substrate 100 and directly contacting thehandling substrate 100. Consequently, in the method according to thepresent invention, a separation space is defined in at least some regionbetween the handling substrate 100 and the polymer substrate 120.Particularly, the separation space may be defined in at least someregion between the upper surface of the convex portion 106 of theconcave-convex structure formed on the handling substrate 100 and thelower surface of the polymer substrate 120.

Thus, due to reduced adhesion between the handling substrate 100 and thepolymer substrate 120, the polymer substrate 120 can be easily separatedfrom the handling substrate 110 and transferred to a transfer medium 200in the course of bringing the polymer substrate 120 into contact withthe transfer medium 200 such as an elastomeric stamp or an adhesive filmand separating the polymer substrate 120 therefrom, as shown in FIG. 6.

In accordance with another aspect, the present invention provides asubstrate for transfer printing fabricated by the method as describedabove. The substrate for transfer printing includes a handling substratehaving a concave-convex structure formed thereon; and a polymersubstrate placed on the concave-convex structure and having bumpsfilling a concave portion of the concave-convex structure of thehandling substrate.

In one embodiment, the concave-convex structure may have a concaveportion having a pot-shaped longitudinal cross-section, an inlet ofwhich is smaller than a belly thereof, and the bumps of the polymersubstrate have a shape corresponding to the shape of the concave portionof the concave-convex structure.

In one embodiment, a separation space may be defined in at least someregion between the handling substrate and the polymer substrate.

In one embodiment, a separation space may be defined in at least someregion between an upper surface of the convex portion of theconcave-convex structure and a lower surface of the polymer substrate.

In one embodiment, the polymer substrate may be comprised of substratesdivided from each other in a predetermined pattern, and each of thedivided substrates may be caught by the concave portion of theconcave-convex structure via by the bumps such that arrangement of thedivided substrates can be maintained.

In one embodiment, a water-soluble polymer may be interposed between thehandling substrate and the polymer substrate at least within the concaveportion of the concave-convex structure to act as an interface adhesionmaterial between the handling substrate and the polymer substrate.

In accordance with a further aspect, the present invention provides amethod for fabricating an electronic device for transfer printing. Themethod for fabricating an electronic device for transfer printingincludes: preparing a handling substrate having a concave-convexstructure formed thereon; forming a sacrificial layer along theconcave-convex structure on the handling substrate; coating a polymer onthe handling substrate having the sacrificial layer formed thereon toform a polymer substrate having bumps filling a concave portion of theconcave-convex structure; and removing the sacrificial layer from thehandling substrate, wherein an electronic device is formed on thepolymer substrate before or after removing the sacrificial layer.

In consideration of conditions for removing the sacrificial layer andconvenience in manufacture of the device, the operation of forming thedevice on the polymer substrate may be suitably determined before orafter removing the sacrificial layer. For example, when the sacrificiallayer is removed using an etching solution such as hydrogen fluoride HF,which can damage the device, it is preferable to form the device afterremoving the sacrificial layer. However, when a water soluble polymer isused as the material for the sacrificial layer and water is used as theetching solution for the sacrificial layer, removal of the sacrificiallayer may be performed after forming the device, since there is noproblem of damaging the device by the etching solution. Since theadhesion between the handling substrate and the polymer substrate isstronger in the case in which the sacrificial layer is not removed thanin the case in which the sacrificial layer is removed, the device can bemore easily formed on the polymer substrate without loss of the degreeof alignment of the polymer substrate when the sacrificial layer is notremoved.

In accordance with a further aspect, the present invention provides amethod of transfer printing a polymer substrate (or an electronic deviceformed on the polymer substrate together with the polymer substrate) toa target substrate using a polymer for transfer printing

As described above with reference to FIG. 6, a polymer substrate 120transferred to a transfer medium 200 may be finally printed onto atarget substrate 300, for example, substrates of plastic, paper, tape,fabric, and other materials, as shown in FIG. 7. Further, since thepolymer substrate 120 has high flexibility, the target substrate 300 mayinclude not only a flat substrate but also a round substrate. Here, inorder to allow the polymer substrate 120 to be easily printed from thetransfer medium 200 to the target substrate 300, a surface 310 of thetarget substrate 300 to be printed may exhibit high adhesive strength,and a separate adhesive material may be deposited on the surface of thetarget substrate in order to enhance adhesive strength.

Meanwhile, before transferring the polymer substrate 120 to the transfermedium 200, a desired device may be previously formed on the polymersubstrate 120, as described above. Thus, even when the target substrate300 is vulnerable to physical and chemical environments in the processof directly forming the device on the target substrate 300, the devicemay be easily formed on the target substrate 300 without being affectedby the properties of the target substrate 300.

Next, the present invention will be described with reference topreferred examples. However, it should be understood that these examplesare provided for illustration only and are not intended to limit thescope of the present invention.

Example 1 Preparation and Transfer Substrate for Transfer Printing

Preparation of Handling Substrate Having Concave-Convex StructureThereon

a) A silicon (Si) substrate was cleaned sequentially using acetone,isopropyl alcohol (IPA) and deionized water, followed by drying at 110°C. for 1 minute. HMDS (AZ AD Promoter-K, 4000 rpm, 35 sec.) and aphotoresist (PR) (AZ 1512, 4000 rpm, 35 sec.) were sequentially coatedon the silicon substrate, and the photoresist was subjected topatterning through a chromium mask (Supermask Co., Ltd.) by 365 nmoptical lithography, followed by development with a water-based basicdeveloper (AZ 500 MIF, AZ Electronics Materials) to form a photoresisthaving a hole pattern.

b) The silicon substrate was etched by reactive ion etching (RIE; VacuumScience, 50 mTorr, 40 sccm SF₆, 50 W, 7 min.), and the remainingphotoresist was removed using acetone and piranha solution(H₂SO₄:H₂O₂=3:1, 5 min.) to form a concave-convex structure pattern onthe surface of the silicon substrate.

Formation of Sacrificial Layer

c) A 200 nm thick SiO₂ layer was deposited on the silicon substratehaving the concave-convex structure by sputtering (Korea Vacuum Tech.,Ar=15 sccm, 5 mTorr, 100 W) or PECVD (Oxford, gas flow: SiH₄=160 sccm,N₂O=730 sccm, 300° C.).

Formation and Patterning of Polymer Substrate

d) An SU-8 photoresist (MICRO CHEM, SU-8 2002, SU-8 2010 or SU-8 2100)was deposited to a thickness of about 14 μm by spin coating at 3000 rpmfor 10 seconds, followed by baking at 95° C. for 1 minute. Then, theSU-8 photoresist was subjected to pattering by UV-lithography (CA-6M,SHINU MST, Illumination: 8.5 mW/cm², 5 sec.), followed by baking at 95°C. for 1 minute and development for 30 seconds with a water-based basicdeveloper (MICRO CHEM, SU-8 developer) to form a patterned polymer(SU-8) substrate.

Removal of Sacrificial Layer

e) The SiO₂ layer was removed by etching with a HF solution (DC ChemicalCo. Ltd, HF 49%, 60 min.).

Transfer

f) A PDMS (polydimethylsiloxane, Sylgard 184, Dow Corning Co., Ltd.)stamp was brought into contact with the polymer (SU-8) substrate andthen separated from the handling substrate (silicon substrate) totransfer the polymer substrate to the PDMS stamp.

FIG. 8 is a schematic diagram of a process of manufacturing a substratefor transfer printing according to Example 1 and SEM images of resultingproducts

Referring to FIG. 8, it can be seen that a concave portion having apot-shaped longitudinal cross-section, an inlet of which is slightlysmaller than a belly thereof, is formed on the handling substrate(silicon substrate) (b), and that the SiO₂ layer was formed along theconcave-convex structure to act as the sacrificial layer (c˜e). Inaddition, it can be seen that the polymer (SU-8) substrate was supportedby the bumps formed on the lower surface thereof and inserted into theconcave portion of the handling substrate, whereby the polymer substratewas maintained at an original place even after the sacrificial layer(SiO₂ layer) was removed (e). Further, it can be seen that the polymersubstrate was easily transferred from the handling substrate to the PDMSstamp only by Van der Waal's force with the PDMS stamp (f, g). Further,the polymer substrate has the bumps having the shape corresponding tothe concave portion of the handling substrate (e,h).

Example 2 Measurement of Alignment and Transfer Yield

Various concave-convex structures were formed by changing the shape,depth and pitch of the concave portion of the concave-convex structureaccording to the procedure of Example 1.

FIGS. 9 and 10 are graphs depicting degree of alignment (AD) andtransfer yield (TY) of a polymer substrate depending on the shape, depth(d) and pitch of a concave portion of a concave-convex structure (here,AD=(Number of polymer substrates on handling substrate after removal ofsacrificial layer/Number of polymer substrates before removal ofsacrificial layer)×100; and TY=(Number of polymer substrates transferredto PDMS/Total number of polymer substrates on handling substrate beforebeing transferred to PDMS)×100). Images on the graphs were obtained byphotographing the handling substrate having the concave-convex structurethereon and the bumps of the polymer substrate.

FIG. 9 shows the degree of alignment and the transfer yield when theconcave portion of the concave-convex structure has a circular shape (intop view of the concave portion). From FIG. 9, it can be seen that, asthe pitch decreases and the depth increases, the degree of alignmentincreases but the transfer yield decreases due to increased adhesionforce between the concave portion of the handling substrate and thepolymer substrate. However, both the degree of alignment and thetransfer yield were 100% at least at a pitch of 20 μm for a depth of 2.8μm, at a pitch of 30˜40 μm for a depth of 4.0 μm, and at a pitch of30˜50 μm for a depth of 4.8 μm.

FIG. 10 shows the degree of alignment and the transfer yield when theconcave portion of the concave-convex structure has a cross shape (intop view of the concave portion), which provides similar tendency to thecase in which the concave portion has a circular shape. However, it canbe seen that both the degree of alignment and the transfer yieldsatisfied 100% in a wider range of the pitch (at a pitch of 30-80 μm fora depth of 2.9 μm and at a pitch of 50-100 μm for a depth of 4.0 μm) inthe case where the concave portion has a cross shape than in the casewhere the concave portion has a circular shape. That is, it can be seenthat the cross-shaped concave portion is more geometrically efficient interms of degree of alignment and transfer yield than the circularconcave portion.

Thus, it is possible not only to adjust the degree of alignment and thetransfer yield to desired values while securing a complete value of 100%in both the degree of alignment and the transfer yield through controlof the shape of the concave-convex structure.

Meanwhile, although the analysis in this example is based on thetransfer yield by the PDMS stamp, the transfer yield may be furtherenhanced when using a stronger adhesive transfer medium.

Example 3 Transfer Printing to Target Substrate to be Printed

Polymer substrates were formed in patterns having various shapes andsizes on handling substrates according to the procedure of Example 1.Then, the polymer substrates were transferred to transfer mediums,followed by printing the polymer substrates on various targetsubstrates.

FIG. 11 shows optical micrographs of a polymer substrate (a) transferredto a transfer medium and polymer substrates (b˜e) printed on targetsubstrates

As shown in FIG. 11, the polymer substrates patterned to various sizesand shapes were transferred to a roller stamp with PDMS attached thereto(a), and it can be seen that the polymer substrates were printed on aflexible substrate such as a PET film while maintaining a high degree ofalignment (b). This shows that the present invention can be applied to aroll-to-roll process for a large area of devices. Further, since thepolymer substrates can be printed along a round shape of a targetsubstrate without deteriorating the degree of alignment (c˜e), thisresult suggests applicability to various types of substrates havingnon-planar structures.

Example 4 Preparation and Transfer Printing of Organic Light EmittingDevices

After preparing polymer substrates in an array of 8×8 according to theprocedure of Example 1, organic light emitting devices were fabricatedon a polymer substrate. Then, the organic light emitting devices weretransferred to a PDMS stamp and then printed on a flexible plasticsubstrate.

FIG. 12 is a picture of organic light emitting devices prepared inExample 4. As shown in FIG. 12, all of the organic light emittingdevices in the pixels were transfer-printed while maintaining an initialdegree of alignment and emitted light upon application of electriccurrent. In other words, the substrate for transfer printing accordingto the present invention allows the devices to be previously formed onthe polymer substrate and to be easily transfer-printed to a targetsubstrate, thereby preventing a possibility of damaging the targetsubstrate in a process of manufacturing the devices.

Further, in the procedure of this example, since the sacrificial layeris removed before manufacturing the devices, there is no damage of thedevices due to a corrosive etching solution (for example, HF solution)used in removal of the sacrificial layer.

Example 5 Preparation and Transfer Printing of Devices for TransferPrinting

A procedure similar to that of the Example 1 was performed. In thisexample, a water soluble polymer (PAA or dextran) was deposited as amaterial for a sacrificial layer on a silicon substrate having aconcave-convex structure thereon by spray coating or spin coating, andZnO-TFTs were formed on the polymer substrate before removal of thesacrificial layer. Then, the sacrificial layer was removed using wateras an etching solution.

Further, after transferring the polymer substrate and the ZnO-TFTs tothe PDMS stamp, the polymer substrate and the ZnO-TFTs were printed ontoa target substrate.

FIG. 13 is a schematic flow diagram of a process of manufacturing andtransferring an electronic device for transfer printing in Example 5.

As shown in FIG. 13, when the water soluble polymer (PAA or dextran) wasused as the material for the sacrificial layer (b), water may be used asan etching solution to facilitate removal of the sacrificial layer(d˜e). Thus, when the devices are formed before the sacrificial layer isremoved (c˜d), water is used as the etching solution, thereby preventingthe devices from being damaged in the course of etching the sacrificiallayer. Further, after the transfer-printing process, the remaininghandling substrate may be reused by washing with a piranha solution andwater (f˜e).

FIG. 14 shows SEM images of resulting products in the respective stepsof the process in Example 5.

In FIG. 14, (a) shows a concave-convex structure having a cross-shapedconcave portion (pitch=20 μm, width=13 μm, depth=3.9 μm) formed on asilicon substrate, (b) shows a water soluble polymer layer formed alongthe concave-convex structure, and (c) shows a ultrathin polymersubstrate formed on the silicon substrate on which the water solublepolymer layer was formed.

In FIG. 14, (d) shows the concave portion before removing the watersoluble polymer layer (sacrificial layer) (left side) and after removingthe water soluble polymer layer (right side). It can be seen that thewater soluble polymer remained inside and around the concave portion dueto phase confinement effects even when the water soluble polymer layerwas removed. Further, it can also be seen that the polymer substratecould be easily transferred from the handling substrate to the PDMSstamp (e), and that the polymer substrate is provided with bumps havingthe shape corresponding to the shape of the concave portion of thehandling substrate (f).

Example 6 Measurement of Degree of Alignment and Transfer Yield

In the procedure of Example 5, experiments have been conducted todetermine optimal conditions for the degree of alignment and transferyield according to coating of the sacrificial layer (in this procedure,the step of forming ZnO-TFTs was eliminated).

Dextran (MW=70,000 g/mol, 2 wt % aqueous solution) and PAA (MW=50,000g/mol, 2 wt % aqueous solution, PAA solution was neutralized with NaOHbefore use) were deposited in various amounts on the silicon substratehaving a cross-pattern by spray coating and spin coating

FIGS. 15 a and 15 b are graphs depicting the degree of alignment (AD)and the transfer yield (TY) measured in Example 6.

When dextran was deposited in an amount of 0.01 mg/cm²˜0.08 mg/cm² asthe material for the sacrificial layer on the silicon substrate by spraycoating, the sacrificial layer could not completely cover the siliconsubstrate. In this case, TY was 0 or very low. This is because thesilicon substrate directly contacted the polymer (SU-8) substrate toexhibit strong adhesion at a portion free from the sacrificial layer,thereby making it difficult to separate the polymer substrate from thesilicon substrate.

When dextran was deposited in an amount of 0.20 mg/cm² on the siliconsubstrate by spray coating, most interior space of the concave portionformed on the silicon substrate was filled with the sacrificial layer.In this case, AD approached 0. This is because the interior space of theconcave portion was filled with the dextran used in an excessive amountand the bumps corresponding to the concave portion were not formed onthe polymer (SU-8) substrate, so that the phase confinement effects weredamaged, thereby allowing the polymer substrate to be deviated from anoriginal place upon removal of the sacrificial layer.

When dextran was deposited in an amount of 0.11˜0.13 mg/cm² by spraycoating, both AD and TY were 100%. Further, when PAA was deposited in anamount of 0.05 mg/cm²˜0.06 mg/cm² by spray coating, both AD and TY were100% (FIG. 15 a).

Meanwhile, when dextran and PAA were spin-coated in amounts of 0.30mg/cm²˜0.41 mg/cm² and 0.19 mg/cm²˜0.21 mg/cm², respectively, both ADand TY were 100% (FIG. 15 b).

Example 7 Measurement of Degree of Alignment and Transfer Yield

In the procedure of Example 5, experiments have been conducted todetermine optimal conditions for the degree of alignment and thetransfer yield according to etching of the sacrificial layer (in thisprocedure, the step of forming ZnO-TFTs was eliminated).

When the sacrificial layer was etched for a period of less than 15minutes, AD was 100% but TY was low. However, when the sacrificial layerwas sufficiently etched for a period of 30 minutes or more, both AD andTY were 100%, and when the sacrificial layer was etched for 5 hours, thesame results were obtained.

On the other hand, when a handling substrate free from theconcave-convex structure was used as a comparative example, AD and TYcould not reach 100% irrespective of etching duration (ComparativeExample 1).

Results of Example 1 and Comparative Example 1 are shown in FIG. 16.

Example 8 Transfer Testing

In the procedure of Example 5, the polymer substrates were formed inpatterns of various sizes and shapes on the handling substrate. Then,the polymer substrates were transferred to a transfer medium (PDMSstamp).

FIG. 17 shows optical micrographs of polymer substrates transferred totransfer media in Example 8. As shown in FIG. 17, it can be seen thatthe polymer substrates patterned to various sizes and shapes werecompletely transferred without changing the degree of alignment.

Example 9 Preparation and Characteristics Analysis of ZnO-TFT

In the procedure of Example 5, ZnO-TFTs were prepared by sequentiallydepositing aluminum (thickness=100 nm) for a source electrode and adrain electrode, ZnO (thickness=80 nm) for a channel layer, c-PVP(thickness=200 nm) for an insulation layer, and aluminum (thickness=100nm) for a gate electrode on a polymer (SU-8) substrate (thickness=11μm). Then, the prepared ZnO-TFTs were transfer-printed together with thepolymer substrate onto a target substrate, followed by observation ofelectric characteristics of the device.

FIG. 18 shows pictures and graphs depicting characteristics of devicesfor transfer printing, which includes a ZnO-TFT array according toExample 9.

In FIG. 18, (a) shows the ZnO-TFT array and a ZnO TFT unit formed on thesilicon substrate, and (b) shows the ZnO-TFT array printed on a sheet ofadhesive paper after being transferred to a PDMS stamp. (c) and (d) arepictures of applicability to a stick-and-play system, in which theZnO-TFT array was transferred to a temporary sticker (c), and thesticker was attached to a rounded object (d).

FIG. 18 (e) and (f) are graphs depicting electrical performance of theZnO-TFTs transfer-printed onto a sheet of paper. A ZnO-TFT formed on anultrathin polymer substrate (thickness: 10 μm) had an electron mobilityof 0.11 cm²/V-sec. This mobility is similar to the electron mobility ofa ZnO-TFT formed on a 200 μm thick polymer substrate, which has beenstudied in other research groups. Meanwhile, the devices had a thresholdvoltage of 5.6V, and an On/Off voltage ratio of 10⁴ upon application of15V (V_(DS)), which was 10 to 100 times higher than devices directlyformed on a plastic substrate.

Further, the ZnO-TFTs were transfer-printed onto a PET (polyethyleneterephthalate) film, followed by observation of characteristics of thedevices according to the degree of bending.

FIG. 19 shows a picture of a ZnO-TFT array transfer-printed on a PETfilm and graphs depicting variation of characteristics of the ZnO-TFTarray depending on the degree of bending

As shown in FIG. 19, it was possible to secure high flexibility andhandling stability by transfer-printing the ZnO-TFTs on a transparentPET film (a). As measured before bending the device, the electronmobility, the threshold voltage, and the on/off voltage ratio of thedevices were 0.09±0.025 cm²V⁻¹S⁻¹, 5.4±0.6 V, and (1±0.72)×10⁵,respectively. The characteristics of the devices were observed in thecase in which the curvature of radius of the devices was graduallychanged from −4.2 mm (compressed state) to 4.3 mm (extended state) (b),and in the case in which the devices were repeatedly subjected 10000times to extension to 4.3 mm and returning back to a planar state. Asthe electrical characteristics of the devices, a relative mobility wasless than 22%, variation of the threshold voltage ranged from −0.7 to0.35V, and the on/off voltage ratio was 10⁴ or more (c, d). Accordingly,it can be seen that performance of the devices could be stablymaintained despite the radius of curvature and repeated bending. Here,device damage or isolation of charges can occur due to penetration ofmoisture and oxygen into the gate insulation layer in the course ofrepeated bending, whereby performance of the devices can be changed.

Although some embodiments have been described herein, it should beunderstood by those skilled in the art that these embodiments are givenby way of illustration only, and that various modifications, variationsand alterations can be made without departing from the spirit and scopeof the invention.

1. A method for fabricating a substrate for transfer printing,comprising: preparing a handling substrate having a concave-convexstructure formed thereon; forming a sacrificial layer along theconcave-convex structure on the handling substrate; coating a polymer onthe handling substrate having the sacrificial layer formed thereon toform a polymer substrate having bumps filling a concave portion of theconcave-convex structure; removing the sacrificial layer from thehandling substrate.
 2. The method according to claim 1, furthercomprising: patterning the polymer substrate to divide the polymersubstrate before removing the sacrificial layer.
 3. The method accordingto claim 1, wherein the concave-convex structure is formed by etchingthe handling substrate through reactive ion etching.
 4. The methodaccording to claim 1, wherein the concave portion of the concave-convexstructure has a pot-shaped longitudinal cross-section, an inlet of whichis smaller than a belly thereof.
 5. The method according to claim 1,wherein the sacrificial layer formed on an upper surface of the convexportion of the concave-convex structure is thicker than the sacrificiallayer formed on a lower surface of the concave portion of theconcave-convex structure.
 6. The method according to claim 1, whereinthe coating a polymer is performed using a solution process.
 7. Themethod according to claim 1, wherein the removing the sacrificial layeris performed by wet etching.
 8. The method according to claim 1, whereinthe sacrificial layer is composed of a water soluble polymer, and theremoving the sacrificial layer is performed using water.
 9. The methodaccording to claim 8, wherein, after removing the sacrificial layer, thewater soluble polymer remains at least within the concave portion of theconcave-convex structure.
 10. The method according to claim 8, whereinthe removing the sacrificial layer comprises adjusting interfacialadhesion between the handling substrate and the polymer substrate byadjusting a degree of etching the water soluble polymer using water. 11.A substrate for transfer printing fabricated by the method according toclaim 1, comprising: a handling substrate having a concave-convexstructure formed thereon; and a polymer substrate placed on theconcave-convex structure of the handling substrate and comprising bumpsinserted into a concave portion of the concave-convex structure.
 12. Thesubstrate for transfer printing according to claim 11, wherein theconcave portion of the concave-convex structure has a pot-shapedlongitudinal cross-section, an inlet of which is smaller than a bellythereof, and the bumps of the polymer substrate have a shapecorresponding to the shape of the concave portion of the concave-convexstructure.
 13. The substrate for transfer printing according to claim11, wherein a separation space is defined at least in some regionbetween the handling substrate and the polymer substrate.
 14. Thesubstrate for transfer printing according to claim 11, wherein aseparation space is defined at least in some region between an uppersurface of the convex portion of the concave-convex structure and alower surface of the polymer substrate.
 15. The substrate for transferprinting according to claim 11, wherein the polymer substrate is dividedinto a plurality of substrates according to a predetermined pattern, andeach of the divided substrates is bound to the concave portion of theconcave-convex structure by the bumps to maintain arrangement of thedivided substrates.
 16. The substrate for transfer printing according toclaim 11, wherein a water soluble polymer is interposed at least withinthe concave portion of the concave-convex structure between the handlingsubstrate and the polymer substrate and acts as an interfacial adhesivematerial between the handling substrate and the polymer substrate.
 17. Amethod for fabricating an electronic device for transfer printing,comprising: preparing a handling substrate having a concave-convexstructure formed thereon; forming a sacrificial layer along theconcave-convex structure on the handling substrate; coating a polymer onthe handling substrate having the sacrificial layer formed thereon toform a polymer substrate having bumps filling a concave portion of theconcave-convex structure; and removing the sacrificial layer from thehandling substrate, wherein an electronic device is formed on thepolymer substrate before or after removing the sacrificial layer.
 18. Asubstrate for transfer printing fabricated by the method according toclaim 2, comprising: a handling substrate having a concave-convexstructure formed thereon; and a polymer substrate placed on theconcave-convex structure of the handling substrate and comprising bumpsinserted into a concave portion of the concave-convex structure.
 19. Asubstrate for transfer printing fabricated by the method according toclaim 2, comprising: a handling substrate having a concave-convexstructure formed thereon; and a polymer substrate placed on theconcave-convex structure of the handling substrate and comprising bumpsinserted into a concave portion of the concave-convex structure.
 20. Asubstrate for transfer printing fabricated by the method according toclaim 3, comprising: a handling substrate having a concave-convexstructure formed thereon; and a polymer substrate placed on theconcave-convex structure of the handling substrate and comprising bumpsinserted into a concave portion of the concave-convex structure.